1. Field of the Invention
The present invention relates to a light emitting device which has improved external quantum efficiency, and in particular, to a light emitting device which can be effectively applied to a surface light source for a full color display, a backlight, an illumination light source or the like; or a light source array for a printer or the like.
2. Description of the Related Art
An organic light emitting device (hereinafter, referred to as an “organic EL device” in some cases) is composed of a light-emitting layer or a plurality of functional layers containing a light-emitting layer, and a pair of electrodes sandwiching these layers. The organic EL device is a device for obtaining luminescence by utilizing at least either one of luminescence from excitons each of which is obtained by recombining an electron injected from a cathode with a hole injected from an anode to produce the exciton, or luminescence from excitons of other molecules produced by energy transmission from the above-described excitons.
Heretofore, a light emitting device has been developed by using a laminate structure from integrated layers in which each layer is functionally differentiated, whereby brightness and device efficiency are remarkably improved. For example, it is described in “Science”, vol. 267, No. 3, page 1332, (1995) that a two-layer laminated type device obtained by laminating a hole transport layer and a light-emitting layer also functioning as an electron transport layer; a three-layer laminated type device obtained by laminating a hole transport layer, a light-emitting layer, and an electron transport layer; and a four-layer laminated type device obtained by laminating a hole transport layer, a light-emitting layer, a hole blocking layer, and an electron transport layer have been frequently used.
However, many problems still remain for putting light emitting devices to practical use. First, there is a need to attain a high external quantum efficiency, and second, there is a need to attain a high driving durability. Particularly, deterioration of quality during continuous driving is one of the most prominent problems.
For example, there has been disclosed in JP-A No. 2003-123984 an attempt to dispose an interface layer of 0.1 nm to 5 nm as a barrier layer between a light-emitting layer and a hole transport layer and retard the migration of holes, to thereby control the migration balance between holes and electrons and enhance the external quantum efficiency. However, this means potentially involves a problem of lowering the brightness and increasing the driving voltage since the migration of all of the carriers is lowered, as well as a problem of lowering the driving durability, since the time that the carriers stay in the device is made longer.
Further, a configuration in which light emitting units each containing a light-emitting layer and a functional layer are stacked in a multi-layer structure is known. For example, JP-A No. 6-310275 discloses a configuration in which plural light emitting units including an organic electroluminescence device are isolated by an insulation layer, and opposing electrodes are provided for each of the light emitting units. However, in this configuration, since the insulation layer and the electrode between the light emitting units hinder the taking out of light emission, light emitted from each of the light emitting units cannot substantially be utilized sufficiently. Further, this is not a means for improving the low external quantum efficiency inherent to each of the light emitting units.
JP-A No. 2003-45676 discloses a multi-photon type organic EL device, in which a plurality of light-emitting layers are isolated from each other by an electrically insulating charge generation layer. However, in this configuration as well, the light emitting units are merely stacked in a plurality, and this cannot provide a means for improving the low external quantum efficiency inherent to each of the light emitting units.
Compatibility between high external quantum efficiency and high driving durability is extremely important for designing a light emitting device which is practically useful, and this is a subject for which improvement is continuously demanded.